Table 2 lists the limitations of conventional cement-bond logs. In an evolution similar to that of openhole acoustic logs, new designs were subsequently introduced that measured signal amplitude at knterpretation near receiver and a full waveform from a far receiver.

Acoustic energy is reflected at interfaces that correspond to changes in acoustic impedance the product of acoustic velocity and density. Good cement bond to casing produces a rapid damping higher impedance of this resonance; poor cement bond results in longer resonance decay lower impedance.

Use this section to list papers in OnePetro that a reader who wants to learn more should definitely read. Straight lines in the CBL waveform, along with high amplitude readings, indicate poor cement-to-casing bond. In the second scenario, good bond, cement is properly bonded to casing and to the formation.

Energy is transmitted at one pad and is received at chl adjacent pad. Channeling is significant because it prevents a hydraulic seal. There is poor cement condition between X and X depth units. This condition is indicated by the lack of later-arriving formation energy.

For example, foam cements, which intentionally interpretatiob void spaces in the cured cement, interpretatiln be misinterpreted as partial bond if normal cement is assumed. Conventional cement-bond logs CBLs comprise a pulsed transmitter and several receivers of acoustic energy positioned as a vertical array of transducers. Tool eccentering reduces signal interpretatlon and travel time Fig. New interpretation methods integrate ultrasonic and attenuation measurements from conventional tools to provide improved cement evaluation in these conditions.

This map can indicate the presence of channels and their orientations. Centering in the cased hole is more critical because the higher-operating frequencies i. The result is that little energy is transferred to the annular fluid and virtually none is transferred to the formation.

CEMENT BOND LOG INTERPRETATION

Acoustic logs provide the primary means for evaluating the mechanical integrity and quality of the cement bond. By analyzing the entire waveform, an acoustic-impedance map of the cement can be constructed.

Pressuring the casing improves the acoustic coupling to the formation and the casing signal will decrease and the formation signal will become more obvious Fig. In this example, the partial bonding is sufficient to provide hydraulic isolation. The recorded waveforms are presented together with the travel time and a casing-amplitude curve, which displays the amplitude of the acoustic signal that has traveled through the casing but not through the cement and formation.

On the basis of empirical data, the log can be calibrated directly in terms of cement compressive strength. Dark zones are the most strongly bonded.

Use this section to provide links to relevant material on websites other than PetroWiki and OnePetro. Fixed gates are set generally at the wellsite to open, remain open, and close at designated times; opening time for the gate is a function of the casing size and the borehole-fluid velocity.

Courtesy of Baker Atlas.

Track 1 contains the gamma ray for correlation and acoustic travel time for quality control. A “straight” waveform display is traditionally interpreted to mean no cement bonding.

Cement bond logs

In addition, each tool design also provides a conventional 5-ft VDL waveform measurement to provide information about the cement-to-formation bond. Channels as narrow as 1. One caveat regarding the use of the amplitude curve for bond evaluation: The second reflection may be at the outside of the casing.

This has the effect of improving the signal strength of both the casing and cement-to-formation arrivals, respectively. In addition to specialized cement-bond devices, modern openhole array tools are designed to also provide conventional cement-evaluation measurements in cased hole.

Track 2 contains the acoustic attenuation logs for the six pads. Track 6 provides the average, minimum, and maximum values of the pipe thickness that is the image shown in Track 7, where red indicates pipe thinning and blue indicates pipe thickening. At the same time, the annulus outside the cement sheath provides poor acoustic coupling.

These tools typically operate at higher frequencies than conventional openhole tools—between 20 and 30 kHz. In contrast, a microannulus a small gap between the casing and cement sheath may extend over long sections of casing but may not prevent a hydraulic seal. These tools use one or more azimuthally sensitive transducers to evaluate cement quality around the circumference of the casing.

Ultrasonic tools can be operated to address two objectives concurrently: Floating gates remain open, but recording is only triggered by an amplitude value greater than a designated threshold value.

The data for circumferential inspection of the casing, as described above, and for the evaluation of cement bonding are obtained on the same logging pass. However, in foamed cements or when exotic additives are used, these calibrations can be inaccurate. If the initial log run was not under pressure and the log indicates poor bond, the presence of a microannulus can be evaluated by running a second bond log under pressure to see if there is a difference.

Part of the problem is that conventional CBL transducer arrays are vertical, whereas bonding problems need to be investigated circumferentially. In the last scenario, partial bond, a space exists within an otherwise well-bonded casing. The cement-evaluation presentation includes casing ovality and tool eccentricity in Track 1. These devices measure the attenuation between two transmitters and receivers as a way of eliminating, or at least minimizing, the effects of:.

However, if only channeling exists, pressuring the casing will not significantly change the log. At greater depths, the pipe is well bonded.

Transmitting elements and the firing sequence are controlled to direct steer and enhance the acoustic-energy output of both the pad transmitters and the VDL transmitter. Positive waveform amplitudes are shown as dark bands and negative amplitudes as gray or white bands; contrast is proportional to amplitude.

However, the presence of gas in cement slurries, as an inert component or as contamination, may seriously affect ultrasonic-tool interpretation. Cement-bond logging tools use gated systems to measure the specific parts of the acoustic waveform needed for the primary bond-amplitude measurement.